1. Field of the Invention
The present invention relates to an exposure apparatus, an exposure method, and a method for producing a device in which a substrate is exposed with a pattern in a state that a liquid immersion area is formed between a projection optical system and the substrate.
2. Description of the Related Art
Semiconductor devices and liquid crystal display devices are produced by the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to form a liquid immersion area so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases).
The conventional technique as described above involves the following problems. The conventional technique is effective because the liquid immersion area can be formed between the projection optical system and the substrate when the scanning exposure is performed while moving the substrate in a predetermined direction. However, the conventional technique is constructed such that the liquid is supplied at a position in front of the projection area onto which the image of the pattern of the mask is to be projected. The conventional technique is constructed such that the liquid is allowed to flow in one direction along with the movement direction of the substrate from the position in front of the projection area. Further, the conventional technique is constructed such that the position (nozzle), from which the liquid is supplied, is also switched when the movement direction of the substrate is switched from the predetermined direction to the opposite direction. However, when the switching operation is performed, then the supply of the liquid in one direction is suddenly stopped with respect to the projection area, and the supply of the liquid in another direction is started. Therefore, the following fact has been progressively elucidated. That is, a problem arises in some cases such that the vibration of the liquid (so-called the water hammer phenomenon) is generated between the projection optical system and the substrate, and the vibration is generated in the liquid supply unit itself (for example, the supply tube and the nozzle). As a result, the deterioration of the pattern image is caused. Further, a problem also arises in other cases such that the liquid immersion area is not formed sufficiently between the projection optical system and the substrate because of the construction in which the liquid is allowed to flow in one direction with respect to the projection area.
The conventional technique as described above also involves the following problem. That is, the liquid cannot be recovered sufficiently in some cases because the recovery unit for recovering the liquid is constructed such that the liquid is recovered on only the downstream side of the liquid allowed to flow in the movement direction of the substrate. If the liquid cannot be recovered sufficiently, it is feared that the liquid may remain on the substrate, and any exposure unevenness may be caused by the remaining liquid. If the liquid cannot be recovered sufficiently, an inconvenience also arises, for example, such that the remaining liquid is scattered to surrounding mechanical parts, and any rust appears. Further, if the liquid remains and/or the liquid is scattered, then the environment (for example, the humidity), in which the substrate is placed, is varied or fluctuated. It is also feared that any desired pattern transfer accuracy cannot be obtained, for example, due to the occurrence of the change of the refractive index on the optical path for the detecting light beam of the optical interferometer to be used to measure the stage position.
When the liquid is recovered from the substrate by using a liquid recovery nozzle, there is such a possibility that the vibration is generated in the liquid recovery unit itself (for example, the recovery tube and the nozzle). If the vibration is transmitted, for example, to the projection optical system, the substrate stage, and/or the optical member of the interferometer for measuring the position of the substrate stage, it is feared that the circuit pattern cannot be formed accurately on the substrate.